1. Field of the Invention
The present invention relates to a magnetic resonance imaging (hereinbelow will be referred to as MRI) apparatus, and more specifically relates to an MRI apparatus which employs an open type super conducting magnet not giving a feeling of pressure to a patient and improves magnetic field stability in a space to be used for inspection.
2. Conventional Art
An MRI, which obtains a tomographic picture image of a human body by making use of nuclear magnetic resonance (hereinbelow will be referred to as NMR) phenomenon, has been broadly utilized in medical facilities. In order to obtain a tomographic picture image correctly reflecting an inner structure of an inspection portion of a body to be inspected, the MRI necessitates a magnet which generates a uniform magnetic field intensity in a space covering the inspection portion. Since a space having a uniform magnetic field can be obtained in a solenoid coil having infinite length, many magnets used in the MRI devices are structured to have a long and narrow cylindrical solenoid coil and a shim structure assembled therein which improves magnetic field uniformity.
Since a magnet structure which requires to position a person to be inspected in a long and narrow space gives a feeling of pressure to the person to be inspected, an MRI apparatus using such magnet is not proper for inspecting such as claustrophobes and children. For this reason, such MRI apparatuses have been developed and become widespread in these days that use a magnet generating a comparatively low magnetic field and structure the magnet such as to provide an opening portion at the side face of the magnet or to broaden a carry-in portion of the person to be inspected at the front face of the magnet.
In order to enhance magnetic field intensity generated by open type magnets, magnets using such as conventional permanent magnets and normal conducting coils as well as assembling super conducting coils have been developed. JP-A-10-179546(1998), JP-A-11-155831(1999) and JP-A-11-197132(1999), for example, show such development. An open type magnet assembling the above referred to super conducting coils can achieve magnetic field intensity generated thereby of 1.0 Tesla, which is five times larger than that generated by a magnet using the conventional permanent magnet and normal conducting coils. Resultantly, with such open type magnet five times larger NMR signal intensity which is substantially comparable with the magnetic field intensity ratio can be obtained, thereby, a sufficient picture quality in case of real time measurement can be ensured.
However, when configuring a cryostat in which super conducting coils are disposed into an open type structure, it is noted that there arises a problem that the cryostat tends to suffer vibration. Namely, in an open type super conducting magnet the cryostat is divided into two parts in each of which a super conducting coil is accommodated in a sealed manner and the divided two parts of the cryostat are arranged to form a space for receiving the person to be inspected therebetween, therefore, differences with regard to vibration for the respective two part divided cryostats are generated and the differences in vibration cause to vary the magnetic field generated by the respective super conducting coils accommodated in the respective two part divided cryostats.
Causes of vibration of the two part divided cryostats are, for example, vibration from a building where the magnet is installed and vibrations of devices themselves such as a gradient magnetic field generating means and a helium refrigerator which are driven in a pulsating manner, for example, with regard to the helium refrigerator helium moves from a compressor to the refrigerator or to the compressor from the refrigerator with a predetermined operating period, therefore, vibration having a predetermined period corresponding to the above operating period is generated. Such periodic vibration causes a pseudo image in a picture image which affects significantly to the picture quality thereof.
Namely, when one of the two part divided cryostats is vibrated with a regular interval (1/xcfx89), measured NMR signals are modulated with oscillation frequency xcfx89. When the modulated signals are processed to convert into such as picture images and spectrums, pseudo images other than a true picture image may appear at respective positions shifted by every xcfx89 as well as pseudo peaks may appear at both sides of true spectrums as sideband waves. Even when the magnetic field variation is microscopic, because of its regular variation the pseudo images and the pseudo peaks appear clearly.
Such a periodic vibration is caused when a helium refrigerator is attached on the cryostat. In order to prevent such vibration, it is tried to introduce a mechanically flexible structural member (bellows) so as not to transfer vibration of the helium refrigerator into the cryostat as disclosed in U.S. Pat. No. 5,363,077.
Further, as disclosed in U.S. Pat. No. 5,952,734, an MRI apparatus, in which a sensor for detecting error magnetic field and a feed back loop control for nulling error components in the sensor signals are incorporated, is proposed.
Although the displacement vibration due to the mechanical vibration of the helium refrigerator can be removed significantly by the flexible structural member (bellows), no effect can be obtained with respect to the displacement due to inertia force. Further, signals due to a pulse operated gradient magnetic field and a high frequency magnetic field which are used for the inspection inherent to the MRI are induced in the magnetic field sensor, it is difficult to detect the microscopic magnetic field (of about 0.05 ppm) due to the mechanical vibration of the helium refrigerator with the magnetic field sensor.
The present invention is completed in view of the above problems, and an object of the present invention is to provide an MRI apparatus which prevents the magnetic field variation due to vibration, in particular, due to vibration caused in an open type high magnetic field MRI apparatus and enhances a reliability of the detection result obtained thereby.
An MRI apparatus according to the present invention which achieves the above object comprises a static magnetic field generating means for generating magnetic field of a predetermined intensity; a gradient magnetic field generating means for generating magnetic field having an intensity gradient; means for generating a high frequency magnetic field; means for detecting nuclear magnetic resonance signals generated from a body to be inspected; and means for processing the nuclear magnetic resonance signals and for displaying the processed result, wherein further comprises a magnetic field correction means for correcting magnetic field variation due to vibration of the static magnetic field generating means, and the magnetic field correction means includes means for generating a correction magnetic field having a frequency corresponding to the frequency of the vibration.
Through the generation of the correction magnetic field having a frequency corresponding to the frequency of the vibration, the magnetic field vibration caused by a periodic vibration is effectively canceled out without detecting the magnetic field variation itself. Thereby, the generation of such as the pseudo images and the pseudo peaks in tomographic picture images due to the vibration is eliminated.
More specifically, in the present invention, the magnetic field correction means is provided with a magnetic field generation coil for generating a correction magnetic field and a power source for driving the magnetic field generation coil, and the power source is inputted of operating signals from a vibration generation source and drives the magnetic field generation coil based on the inputted operating signals.
Further, in one preferred embodiment of an MRI apparatus according to the present invention, the static magnetic field generation means includes a super conducting coil for generating a static magnetic field and a cryostat with a refrigerator into which the super conducting coil is accommodated, and the correction magnetic field generation means corrects the magnetic field variation caused by the vibration generated by the refrigerator.
With the MRI apparatus according to the preferred embodiment, the operating signals for driving the refrigerator are directly taken out from the drive source for the refrigerator and are provided to the correction magnetic field generation means, thereby, a correction magnetic field having a frequency corresponding to the frequency of the vibration is generated and thus, the magnetic field variation caused by the vibration generated by the refrigerator can be canceled out.
The MRI apparatus according to the present invention is particularly applicable to one having such a static magnetic field generation means which is structured to arrange at least a pair of super conducting coils so as to sandwich a space for laying a person to be inspected.